Dipicrylamine as a colorimetric sensor for anions: experimental and computational study

Abstract

Dipicrylamine (2,4,6-2′,4′,6′-hexanitrodiphenylamine, DPA) has been used for the detection and extraction of metal ions, especially potassium; however, its capability as an anion sensor has not been reported to date. It contains a secondary amine (N–H), the proton of which can form H-bonds with anions. This property has been exploited to investigate the capability of DPA as an anion sensor. Out of the large number of anions used in this study, F⁻, OAc⁻, and H₂PO₄⁻ exhibited strong interactions with sharp colour changes in acetonitrile. The DPA-anion recognition event was monitored by UV-vis, NMR and ESMS studies, apart from distinct colour changes detectable by the bare eye. A detailed investigation revealed that the anions first interact with the N–H proton through H-bonding and then deprotonation takes place forming a DPA⁻–TBA⁺ (tetrabutylammonium) complex. The rate constants of these complex formation have been determined from time dependent UV-vis spectral change and the order of the observed rate constants is F⁻ > OAc⁻ > H₂PO₄⁻. For F⁻, the NMR and ESMS data indicated the interaction of F⁻ with one of the carbons or its attached proton in one of the benzene rings. A computational study suggests that the F⁻ ion binds with one of the phenyl carbons instead of the –CH hydrogen bond of DPA⁻.

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Introduction

The development of optical sensors for the selective recognition of anions is an emerging area in chemistry, as anions play an important role in many chemical and biological processes.¹ In recent years, special attention has been given to anions such as fluoride, phosphate, and acetate.² Fluoride has been considered by the US National Academy of Sciences as an essential nutrient: a daily intake of 1–3 mg of fluoride prevents dental caries.^2g However, a long-term intake of higher amounts causes acute toxic effects; even a death has been reported.^2h Phosphate is again considered as an essential nutrient required for critical biological reactions that maintain the normal homoeostatic control of cells. However, excessive retention of phosphate in the body can cause toxicity resulting in a wide range of cellular and tissue injuries.²ⁱ Acetate is a common anion in biology and at low concentrations, generally, not harmful to the body; in fact, some salts of acetates are use as food additives. However, increased acetate levels are harmful to the human body.^2j Therefore, because of clinical and environmental reasons, early detection of these ions in the environment is desirable.² For the detection/estimation of anions, different instrument based analytical and spectroscopic techniques are available.³ All these instrument based techniques require sophisticated analytical instruments: many of these techniques are not simple and suitable for quick and online analysis. Some methods require lengthy processes for sample preparation and some of the methods necessitate a large number of samples. As an alternative, considerable efforts have been expended to develop sensor based colorimetric and fluorescent molecular probes for the recognition of anions.^1,2 Among these, the colorimetric method is more attractive because of its simplicity and bare-eye detection.^1,2,4

For the designing of anion sensors, various noncovalent interactions such as hydrogen-bonding, anion–π and reactions like hydrogen abstraction, electron transfer, etc. are mainly considered.⁵ Molecules containing amide and amine groups, particularly secondary amines having proton(s) with acidic character, can effectively interact with anions and can function as anion sensors.^5,6 Systems with very strong hydrogen bonding sometimes may lead to proton abstraction, which, in turn, exhibit colour changes due to charge transfer (CT) transitions.^5g Electron deficient organic compounds with π-system having a strong positive quadrupole moment can also make effective anion–π interaction functioning as chromogenic sensors.^2d Recently, a number of anion sensors have been reported, the operation of which is based on anion induced single/double electron transfer processes.^5i,j All of these interactions and charge/electron transfer processes lead to perturbation to their photophysical properties, resulting in fluorescent/colour changes allowing the anion to be detected.

Dipicrylamine (2,4,6-2′,4′,6′-hexanitrodiphenylamine, DPA, Fig. 1) contains a secondary amine group, the proton of which is readily deprotonable (pK = 2.62)⁷ and this molecule has been extensively used for the recognition and selective extraction of alkali and alkaline earth metal ions, especially potassium.^8–10 The crystal structures of DPA and computational study on the intermolecular interactions revealed that after deprotonation, the partial delocalization of the resultant negative charge mediated by the aromatic rings facilitates the coordination of the oxygen atoms of the nitro group to suitable metal ions.¹¹ The six nitro groups that are flexible and can interact and adjust in the space facilitates intermolecular interactions to form a network structure encapsulating metal ions in the cavities.¹² Though substantial work on this molecule (DPA) has been performed to investigate its complexation property with metal ions, its interaction with anions has not been reported yet. Since this molecule contains N–H that has the potential to interact with anions through H-bonding, we wanted to exploit this property of DPA to investigate its capability as an anion sensor, which has not been reported so far.

Fig. 1 Structural drawing of dipicrylamine (DPA).

In this paper, we report the interaction of DPA with a wide range of anions in acetonitrile, which revealed strong interactions with F⁻, OAc⁻, and H₂PO₄⁻ with sharp colour changes. A detailed investigation has been made with the aid of UV-vis and NMR spectroscopy, and quantum chemical calculations have been employed to investigate the site of interaction with the anion and to rationalize the experimental observations.

Results and discussion

The interaction of different anions with DPA has been assessed by UV-vis spectral changes and also by colour changes, detectable by bare eyes. The details of the experimental procedure for recording UV-vis spectral changes for various anions are given in the Experimental Section. Out of the ten anions (F⁻, Br⁻, Cl⁻, ClO₄⁻, OAc⁻, I⁻, HSO₄⁻, NO₃⁻, BF₄⁻ and H₂PO₄⁻) used in this study, only three anions, namely, F⁻, OAc⁻ and H₂PO₄⁻, exhibited immediate colour changes along with substantial UV-vis spectral changes, whereas other anions did not exhibit any significant changes either in colour or in UV-vis spectra. We also tested CN⁻ and N₃⁻ in acetonitrile, in which CN⁻ exhibited a slow colour change from yellow to orange with a high concentration of anions, whereas N₃⁻ did not show any change. The UV-vis spectral change was not straightforward; the acetonitrile solution of DPA exhibits a single band at 420 nm (ε = 2.73 × 10⁴). After the addition of TBAF, there was a substantial enhancement of the intensity (40%) of this band with a slight red shift of λ_max to 425 nm and a new strong band grew at 508 nm. This spectral change was completed within 30 s; after that, the spectra gradually changed again and the intensity of the 425 and 508 nm bands decreased with the growing of a new broad band of low intensity around 600 nm showing two clear isosbestic points at 555 and 374 nm. This change continued for almost 5 h. The spectrum of DPA, the intermediate spectrum recorded after 30 s of the addition of TBAF and the final spectrum recorded after 5 h are shown in Fig. 2. The colour change noted at the intermediate stage after the addition of anions and that after 12 h are shown in Fig. 3. For OAc⁻ and H₂PO₄⁻, the UV-vis spectral changes are similar but the rate of change was considerably slow and the final spectrum is different from that of F⁻. For OAc⁻, the intermediate spectrum was optimum after 9 min and for H₂PO₄⁻, it was after 36 min; the final spectra were recorded after 12 and 24 h for OAc⁻ and H₂PO₄⁻, respectively. The final spectrum for OAc⁻ exhibited a weak shoulder around 640 nm, a sharp band at 540 nm and the original band of DPA (420 nm) appeared at 416 nm with low intensity (Fig. 4). For H₂PO₄⁻, the weak shoulder appeared around 620 nm, the new band appeared at 508 nm and the original band of DPA appeared almost at the same position (422 nm) with slightly low intensity (Fig. 5). These observations suggest that all the three anions first interacted with the DPA forming an intermediate compound, which further transformed slowly into a stable compound; the nature of the final spectra also suggest that the end products for OAc⁻ and H₂PO₄⁻ anions are probably similar but they are different from that of F⁻. The time dependent spectral changes from intermediate to final spectra for F⁻ and OAc⁻ are shown in Fig. 6 and 7, respectively, and the same for H₂PO₄⁻ is submitted in the ESI, Fig. S1.† The clean spectral changes with sharp isosbestic points for all the three cases also suggest that there is no side reaction to form any other products.

Fig. 2 UV-vis spectra of DPA in acetonitrile (2.5 × 10⁻⁵ M) before and after the addition of TBAF.

Fig. 3 Colour change of the acetonitrile solution of DPA (2.5 × 10⁻⁴ M) before and after the addition of various anions (2.5 × 10⁻² M), recorded within 20 minutes and after 12 h of addition.

Fig. 4 UV-vis spectra of DPA in acetonitrile (2.5 × 10⁻⁵ M) before and after the addition of TBA (OAc).

Fig. 5 UV-vis spectra of DPA in acetonitrile (2.5 × 10⁻⁵ M) before and after the addition of TBA (H₂PO₄).

Fig. 6 UV-vis spectral change of DPA in acetonitrile (2.5 × 10⁻⁵ M) as a function of time after the addition of TBAF (2.5 × 10⁻³ M).

Fig. 7 UV-vis spectral change of DPA in acetonitrile (2.5 × 10⁻⁵ M) as a function of time after the addition of TBA (OAc) (2.5 × 10⁻³ M).

The UV-vis spectra were also recorded with varying amounts of anions, from 2 to 1000 molar equivalent and it was noted that the pattern of spectral changes are independent of the molar equivalent of anions added but the rate of change enhanced with increasing the amount of anions added. It may be noted that the initial change, which is the enhancement of the intensity of the 420 nm band with a marginal red shift and growing of a new band at 508 nm are similar for all of the three anions, namely, F⁻, OAc⁻ and H₂PO₄⁻ (Fig. 2, 4 and 5), and this change is due to the formation of a hydrogen-bonded complex between N–H of DPA and anions.^2d,4c,e,f The next slow change, which drastically reduced the intensity of the 425 and 508 nm bands and new bands grew in the region from 600 to 650 nm, as mentioned above and shown in Fig. 2, 4 and 5, is assigned to the deprotonation of N–H of DPA and complex formation between DPA⁻ and TBA salts of the anions. As the spectral changes exhibited clean isosbestic points, it may be assumed that there was no significant side reaction(s); on this basis, we tried to fit the data of changes in the absorbance as a function of time to the first order rate equation, ln|A − A_∞| = −kt + ln|A₀ − A_∞|, where A₀ is the initial absorbance and A_∞ is the absorbance when the reaction is completed, and to evaluate the observed rate constant for comparison among three systems. The plot of F⁻ for monitoring the band at 508 nm is shown in Fig. 8 and similar plots for OAc⁻ and H₂PO₄⁻ are included in the ESI, Fig. S2 and S3.† It may be noted that the fittings are very good (R² = 0.998), and the observed rate constant thus calculated at two different wavelengths (424 and 508 nm) are summarized in Table 1. The observed rate constants at two wavelengths are close, and it is in the deceasing order of F⁻ > OAc⁻ > H₂PO₄⁻. This trend is similar to their pK_a values for OAc⁻ and H₂PO₄⁻, 4.8 (OAc⁻) > 2.12 (H₂PO₄⁻); however, F⁻ (3.2) did not follow this order, which is probably due to the reaction of F⁻ with the aromatic ring, which is discussed in detail in the section titled computational study. However, to check whether DPA⁻ interacts with F⁻ or not, K⁺ and Ca²⁺ salts of DPA were prepared and allowed to interact with TBAF in aqueous media, as the DPA⁻ salts are soluble in water. The UV-vis spectral changes for both the salts (Fig. S4 and S5, ESI†) are similar to that of F⁻, which again indicate the interaction of F⁻ with DPA⁻.

Fig. 8 Plot of the first order rate equation to determine the observed rate constant from UV-vis spectral change of DPA as a function of time after the addition of TBAF.

Table 1 Observed rate constant for the reaction of DPA with anions in acetonitrile, calculated at two different wavelengths

Anions	Rate constant (k) s^−1
	424 nm	508 nm
F^−	0.0159	0.0160
OAc^−	0.0102	0.0101
H2PO4^−	0.00118	0.0012

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Mass spectrometry

ESMS is a soft ionisation technique and useful to investigate intermolecular interactions in a solution. It was, therefore, decided to perform mass analysis of the solution upon the addition of anions. This analysis was performed using LC-MS after the addition of anions into the DPA solution following the method described in the experimental section and the data obtained are also given there. For F⁻, two strong peaks were observed at e/m values of 438.33 and 720.85 (Fig. S6†), which correspond to the compound [DPA⁻]⁻ and the compound of composition [DPA⁻ + TBA⁺ + HF₂⁻]⁻, respectively. For OAc⁻ and H₂PO₄⁻, the highest and strong peaks were noted at 1118.36 and 1118.53, respectively, and they correspond to the composition [2DPA⁻ + TBA⁺]⁻ (calculated value: 1118.87). Apart from these peaks, the OAc⁻ anion exhibited two other peaks at e/m 556.61 and 661.76, which corresponds to [DPA + OAc⁻ + HOAc]⁻ and [DPA + OAc⁻ + NaOAc + 2CH₃CN]⁻, respectively; further, the H₂PO₄⁻ ion exhibited four peaks at e/m 534.37, 632.38, 971.78 and 1069.74, (Fig. S7†), which correspond to [DPA + H₂PO₄⁻]⁻, [DPA + H₂PO₄⁻ + H₃PO₄]⁻, [DPA + H₂PO₄⁻ + H₃PO₄ + TBAH₂PO₄]⁻ and [2DPA + H₂PO₄⁻ + H₃PO₄]⁻, respectively. ESMS is a soft ionization technique and the mass of the assembly including solvents associated by weak intermolecular interactions is often observed.¹³ The mass spectroscopy data, therefore, confirmed the interaction of anions with the DPA molecule. In all the three cases, the N–H of the DPA is finally deprotonated to form DPA⁻ and it is stabilized by the interaction with the TBA⁺ cation. In a few cases, as mentioned above, the proton of the amino group of the DPA paired up with anion through N–H⋯O (oxygen from OAc⁻ or H₂PO₄⁻) H-bonding interaction: the other molecules including solvents are probably associated with the assembly by short contact.^2a,b,5,6

IR and ¹H NMR study

The deprotonation of DPA upon interaction with anions was also confirmed from IR and ¹H NMR spectroscopy. The IR spectrum of DPA exhibits a strong and sharp peak at 3242 cm⁻¹ due to ν(N–H), which disappeared when the DPA solution was treated with TBAF. For the investigation of the details of DPA–anion interactions, ¹H NMR study was carried out: details of the experimental procedure and the ¹H and ¹³C NMR data are given in the Experimental section. DPA in acetonitrile exhibited two singlets at δ 9.05 (4H) and 10.61 (1H) due to four equivalent aromatic protons and the N–H proton, respectively. After the addition of TBA⁺ salts of the anions, in all the three cases, the singlet observed for aromatic protons at δ 9.05 was shifted to the upfield region at δ 8.72, 8.72 and 8.985 for F⁻, OAc⁻ and H₂PO₄⁻, respectively. The time dependent NMR spectra for OAc⁻ and H₂PO₄⁻, the first one of which was recorded within minutes after the addition of anions and then after 1, 4 and 24 h, exhibited that for H₂PO₄⁻, the upfield shift observed in the first spectrum recorded upon the addition of an anion remained almost the same up to 24 h (Fig. 9). For OAc⁻, this situation is similar to that of H₂PO₄⁻, except for some small peaks (not well resolved) that grew in the later stages (Fig. S8†), which is probably due to formation of a minor species. However, for F⁻, apart from the upfield shift of the singlet, new singlets at δ 5.28 and 5.02 and a few new peaks in the range from δ 8.51 to 8.72 have grown after the addition of the anion (Fig. 10). The new peaks in this spectrum were growing with time at the expense of the original singlet and it continued for almost 24 h. Beyond that, no further change is noted during the testing period of 96 h. From the NMR data, it is concluded that the shielding of the singlet in all the three cases in the first instant after the addition of anions is due to the deprotonation of the N–H proton of the DPA to form DPA⁻. The negative charge on the nitrogen atom is delocalized on two benzene rings, which resulted in shielding of the protons attached with the benzene rings causing an upfield shift of the singlet. The other important conclusion is that for OAc⁻ and H₂PO₄⁻ anions, the four aromatic hydrogen atoms of the DPA remained equivalent even after the deprotonation of the N–H group; in other words, the DPA molecule remained intact (except deprotonation), but for the F⁻ ion, the four protons are no longer equivalent and at least one of the protons is affected either by some reaction in the benzene ring or by some other strong interaction with some highly electronegative moiety, as the new signal appeared in a substantially high field region, δ 5.28 and 5.02 (Δδ = 3.77 and 4.03 ppm). Since the interaction of N–H with fluoride generates HF₂⁻ (ref. 2a) and it is difficult to observe this signal at room temperature, low temperature (−20 °C) ¹H NMR spectrum was recorded and the signal appearing at 16.05 (Fig. S9†) is assigned to the proton of HF₂⁻.^2a The ¹³C NMR spectra of DPA before and after the addition of anions (F⁻, OAc⁻ and H₂PO₄⁻) were also recorded. The DPA exhibits four signals at δ 117.33, 126.52, 134.55 and 141.77, as expected from the four types of carbon atoms in this molecule. However, after the addition of F⁻, five peaks at δ 118.26, 125.19, 133.09, 139.85 and 143.48 appeared in the aromatic region (Fig. 11), but for OAc⁻ and H₂PO₄⁻ anions, four peaks with a slight change in the chemical shifts as compared to free DPA were noted. Both the ¹H and ¹³C NMR data, therefore, suggest that the F⁻ ion might have interacted with one of the electron deficient carbon or proton attached to the carbon of the aromatic ring and the equivalent nature of the two rings is lost. To investigate this phenomenon, a computational study was undertaken to explain this observation.

Fig. 9 ¹H NMR spectral change of DPA recorded after the addition of TBA (H₂PO₄) in CD₃CN.

Fig. 10 ¹H NMR spectral change of DPA recorded after the addition of TBAF in CD₃CN.

Fig. 11 ¹³C NMR spectral change of DPA recorded after the addition of TBAF in CD₃CN.

Computational study

Quantum chemical calculations have been performed to examine the mode of interactions of the F⁻ ion with the DPA molecule. Their geometries were optimized with RHF/6-31+G* level of theory,¹⁴ and single point energy calculations were performed with B3LYP/6-31+G** and M06/6-31+G** methods.^15,16 The experimental results (vide supra) suggest that the F⁻ ion deprotonates the amine nitrogen instantly, which results into the formation of the DPA anion (1) (Fig. 12). Therefore, the calculations were performed with 1. The DPA anion (1) can interact with anion via –CH⋯anion interactions or/and through anion–π type interactions between the anion and phenyl ring. The possible modes of interactions have been examined in this study. The interactions of anions with electron deficient arenes are reported in the literature.¹⁷ The binding energies calculated with all the studied levels suggest for the repulsive interactions of the F⁻ ion with the –CH hydrogen of the phenyl ring (1-CH–F) (Fig. 12). However, the optimization of the geometry with the F⁻ ion at the centre of the phenyl rings leads to the geometry that shows the interaction of F⁻ ion with the –CH carbon of the phenyl ring (1-top-F). M06/6-31+G**//RHF/6-31+G* results indicate −3.0 kcal mol⁻¹ binding energy for 1-top-F. These calculated results suggest that preferably F⁻ ion binds with phenyl carbon instead of the –CH hydrogen of DPA anion 1 (Fig. 12).

Fig. 12 RHF/6-31+G* optimized geometries, calculated fluoride ion affinities (kcal mol⁻¹) and important distances (Å). B3LYP/6-31+G**//RHF/6-31+G* and M06/6-31+G**//RHF/6-31+G* calculated fluoride ion affinities are given in parentheses and square brackets, respectively (yellow = carbon; blue = nitrogen; red = oxygen; white = hydrogen; cyan = fluoride).

The observed repulsive interaction of F⁻ ion with 1 is presumably due to the interaction between two anions, namely, DPA⁻ anion (1) and F⁻ ion. To circumvent this situation, we have performed the calculation using [DPA anion–TBA] complex (2) (Fig. 13). It is worth noting that in the experimental study, the tetrabutylammonium salt of fluoride ion was used. Further, the mass spectra study also suggests for the complex formation between DPA and TBA anions (vide supra). The calculated binding energies with 2 suggest for an attractive interaction of the F⁻ ion at both the sites, i.e., with –CH hydrogen (2-CH–F) and –CH carbon (2-top-F) of the phenyl ring (Fig. 14). The F⁻ ion interacts with the –CH hydrogen of 1 with −46.3 kcal mol⁻¹ with M06/6-31+G**//RHF/6-31+G* level of theory (2-CH–F); however, F⁻ ion showed a much stronger interaction (−58.3 kcal mol⁻¹) with the –CH carbon of 1 at the same level of theory. These results suggest for the binding of F⁻ ion with –CH carbon more preferentially than the –CH hydrogen.

Fig. 13 RHF/6-31+G* optimized geometry of 2 (yellow = carbon; blue = nitrogen; red = oxygen; white = hydrogen).

Fig. 14 RHF/6-31+G* optimized geometries, calculated fluoride ion affinities (kcal mol⁻¹) and important distances (Å). B3LYP/6-31+G**//RHF/6-31+G* and M06/6-31+G**//RHF/6-31+G* calculated fluoride ion affinities are given in parentheses and square brackets, respectively (yellow = carbon; blue = nitrogen; red = oxygen; white = hydrogen; cyan = fluoride).

The mass spectroscopy study reveals that DPA with tetrabutyl ammonium salt of fluoride can form [DPA⁻ + TBA⁺ + HF₂⁻]⁻ (vide supra). Therefore, there is a possibility for the additional interaction of C–F fluoride with an HF molecule via intermolecular hydrogen bonding. It is likely that HF can form in the solution through the deprotonation of –N–H hydrogen in DPA. We have examined this possibility, and the calculated results reveal that such a binding mode of F⁻ with 1 is much higher (−69.5 kcal mol⁻¹) (2-top-F–HF) at the theoretical M06/6-31+G** level than the previous binding modes (Fig. 14). We have also examined two additional possibilities for the binding of two F⁻ ions with one DPA molecule, i.e., the interaction of two fluoride ions with –CH carbons of each phenyl ring (2-2F-top) and the interaction of one F⁻ ion at –CH carbon and second F⁻ with –CH hydrogen (2-2F-top-CH) (Fig. S10, ESI†). The calculated interaction energies for these two possibilities are lower than 2-top-F–HF (Fig. 14 and S10, ESI†).

Further, to see the influence of the solvent on fluoride ion affinity, single point energy calculations were performed in the acetonitrile solvent medium with B3LYP/6-31+G** and M06/6-31+G** methods using conductor-like polarisable continuum salvation model (CPCM).¹⁸ The calculated fluoride ion affinities in the solvent medium are smaller in energy than the gas phase calculated energies; however, 2-top-F–HF shows the highest binding energies with both the theoretical levels (Table 2). These results are also in support of the preferential binding of F⁻ ion with the –CH carbon of phenyl ring.

Table 2 Calculated fluoride ion affinities (kcal mol⁻¹) in acetonitrile solvent medium using CPCM salvation model

	B3LYP/6-31+G**	M06/6-31+G**
1-CH–F	1.33	−0.15
1-top-F	−0.66	−7.59
2-CH–F	14.17	3.94
2-top-F	−1.60	−10.97
2-top-F–HF	−7.90	−16.32

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Conclusions

The property of dipicrylamine as a colorimetric sensor for anions has been investigated. In acetonitrile, it exhibited strong interactions with F⁻, OAc⁻, and H₂PO₄⁻ out of a large number of anions tested, and the interaction resulted in sharp colour changes. The initial colour changes for all the three anions are similar; however, with time, a distinct colour difference for all the three anions was noted. The DPA–anion interaction was followed by UV-vis, NMR (¹H and ¹³C) and ESMS studies, which revealed that in the first step, anions form H-bonding interactions with the N–H proton of the DPA. Then, the deprotonation of N–H takes place forming a complex between DPA⁻ and TBA⁺. The spectral change for the complex formation is slow, and the observed rate constants for the formation of these complexes have been determined from the UV-vis spectral change and the order of the observed rate is F⁻ > OAc⁻ > H₂PO₄⁻. However, F⁻ interacts with one of the carbons of a phenyl ring, which has been suggested by NMR and ESMS studies: this has been further confirmed by quantum chemical calculations.

Experimental

Materials

Dipicrylamine was prepared in the laboratory following the modified reported procedure.¹⁹ Tetrabutylammonium salts of all the anions were purchased from Sigma-Aldrich and Fluka. All the solvents were purchased from Merck, Fischer Scientific and were purified by a standard procedure before use.

Instrumentation

The UV-vis spectra were recorded on a CARY 500 scan Varian spectrophotometer. NMR spectra were recorded on a model DPX 200 and Advance II 500 MHz Bruker FT-NMR instruments using TMS as the internal standard. Mass spectra were recorded on a Q-TOF Micro™ LC-MS instrument. The infrared spectra were recorded on a Perkin-Elmer spectrum GX FT-system as KBr pellets.

Interaction with anions by UV-vis study

The interaction of DPA with various anions was followed by a UV-vis spectral change and also by the visualization of colour change with bare eyes. In a typical experiment, a stock solution of DPA (5 × 10⁻⁵ M) and that of tetrabutylammonium (TBA) salts (5 × 10⁻³ M) of a series of anions (F⁻, Br⁻, Cl⁻, ClO₄⁻, OAc⁻, I⁻, HSO₄⁻, NO₃⁻, BF₄⁻, and H₂PO₄⁻) were prepared in freshly distilled acetonitrile. Then, 2 mL of the stock solution of DPA and 2 mL of the stock solution of each anion were taken in a 5 mL volumetric flask, so that the effective concentration of DPA was 2.5 × 10⁻⁵ M and that of the anion was 2.5 × 10⁻³ M. The absorbance spectra of DPA (2.5 × 10⁻⁵ M) and the resulting solutions upon the addition of anions were recorded. A significant spectral change was noted for F⁻, OAc⁻ and H₂PO₄⁻, whereas for the other anions, no significant spectral change was noted. The spectral changes for these three ions are interesting and recorded as a function of time till saturation. Details of this are discussed in the Results and discussion section. The colour changes just after the addition of anions and after the completion of the reaction were also photographed.

Mass spectroscopy

For recording the mass spectra, 1 mL stock solution of DPA and 1 mL stock solution of the anions (F⁻, OAc⁻ and H₂PO₄⁻) were mixed and the mass spectra of the resulting solutions were recorded within 10 min of the addition and also after 24 h. Mass data: e/m for F⁻, 438.33, [DPA⁻]⁻, (calculated 438.20) and 720.85, [DPA⁻ + TBA⁺ + HF₂⁻]⁻, (calculated 719.68); for OAc⁻, 438.08, [DPA⁻]⁻, (calculated 438.20) 556.61, [DPA + OAc⁻ + HOAc]⁻, (calculated 558.30); 661.76, [DPA + OAc⁻ + NaOAc + 2CH₃CN]⁻, (calculated 662.40) and 1118.36, [2DPA⁻ + TBA⁺]⁻ (calculated 1118.87) and for H₂PO₄⁻, 534.37, [DPA + H₂PO₄⁻]⁻, (calculated 536.20); 632.38, [DPA + H₂PO₄⁻ + H₃PO₄]⁻, (calculated 634.19); 971.78, [DPA + H₂PO₄⁻ + H₃PO₄ + TBAH₂PO₄]⁻, (calculated 973.65); 1069.74, [2DPA + H₂PO₄⁻ + H₃PO₄]⁻, (calculated 1073.40); 1118.53, [2DPA⁻ + TBA⁺]⁻ (calculated 1118.87).

NMR spectral study

The ¹H and ¹³C NMR spectra of DPA before and after the addition of anions (F⁻, OAc⁻ and H₂PO₄⁻) were recorded in CD₃CN. In a typical experiment, 2 mg of DPA dissolved in 0.5 mL of CD₃CN was added to the tetrabutylammonium salts of the anions (solid, 20 molar equivalents) and the spectra were recorded at different time intervals up to 24 h of addition. The ¹H NMR data (CD₃CN): DPA, δ 10.61 (1H, NH) and 9.05 (4H, Ar–H); after 24 h of addition of anions (only aromatic protons): for F⁻, δ 8.72, 8.61, 8.58, 8.53, 8.51, 5.28 and 5.02; for OAc⁻, δ 8.73 and for H₂PO₄⁻, δ 8.985; ¹³C NMR data (CD₃CN): DPA, δ 117.33, 126.52, 134.55 and 141.77; after 24 h of addition of anions (only aromatic protons): for F⁻, δ 118.26, 125.19, 133.09, 139.85 and 143.48; for OAc⁻, δ 117.03, 123.86, 131.79 and 138.55 and for H₂PO₄⁻, δ 117.41, 124.17, 132.08 and 138.13.

Computational methods

All the geometries were fully optimized in the gas phase with the restricted Hartree–Fock method¹⁴ using the 6-31+G* basis set. To calculate the energies with a higher basis set, single point energy calculations were performed at the B3LYP/6-31+G**M06/6-31+G** levels^15,16 using RHF/6-31+G* optimized geometries. The choice of the M06 method is based on its reliability towards the calculations of non-bonding intercations.¹⁶ Further, single point energy calculations were performed in acetonitrile (ε = 35.688) using conductor-like polarisable continuum salvation model (CPCM).¹⁸ This approach describes the solvent reaction field by means of apparent polarization charge distribution on the cavity surface. All the calculations were performed with the Gaussian 09 suite program.²⁰

Acknowledgements

CSIR-CSMCRI Registration no.: 150. We thank CSIR, New Delhi, for funding this work under the project CSC 0134 (M2D) and for generous support towards the infrastructure and core competency development. R.G. and M.K.K. gratefully acknowledge the CSIR and UGC, respectively, for awarding Senior Research Fellowship (SRF). We thank Dr V. P. Boricha and Mr A. K. Das and Mr V. Agarwal for recording NMR, ESMS and IR spectra, respectively.

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Footnote

† Electronic supplementary information (ESI) available: Fig. S1–S10 (UV-Vis, mass, NMR and computationally optimized structures). See DOI: 10.1039/c4ra09099c

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